The invention herein is directed to an assembly for providing sterile chilled diluent for use in thermodilution procedures for determining cardiac output.
Cardiac output, defined as the volumetric flow rate of blood through the heart (usually in liters per minute), is a critical parameter in determining the state of health of the critically ill patient, especially those who have suffered a significant cardiac insult such as major cardiac surgery or acute myocardial infarction ("heart attack"). Studies have indicated that a typical cardiac patient will have this procedure performed approximately 30 times during their hospitalization.
The earliest method for determining cardiac output was the Fick method which measured the cardiac output value indirectly by measuring the body's oxygen rate of uptake and dividing this by the difference in oxygen concentration in the arterial and nervous sides of the circulatory system. This method had certain built-in inaccuracies and required waiting for the laboratory to determine the blood oxygen concentrations. The cost of analysis and the need for more rapid results led to the development of two invasive dilution techniques, dye dilution and thermal dilution. In both techniques a catheter is inserted into the flow region. In the dye dilution technique, a dye of known concentration is released in the blood stream and is sampled downstream. The amount of dilution is measured using a colorimeter. The flow rate can be calculated using the Stewart-Hamilton equation.
The thermal dilution technique, first introduced in 1971 by Drs. Swan and Ganz, replaced the dye injection with a bolus of fluid which is at a known temperature, significantly lower than the blood temperature and the downstream "thermal" dilution is monitored with a temperature sensor imbedded in the catheter. Because all of the measured variables are electrical in nature, the output from the thermodilution temperature sensor and the temperature of the injectate can be monitored by a small dedicated computer which uses a modified form of the Stewart-Hamilton equation to calculate and display the cardiac output within a minute of the injection of the bolus. This technique has become the method of choice for determining cardiac output because of the ease of use and the rapid display of the test results.
The modified Stewart-Hamilton equation is of the form: ##EQU1## where: ##EQU2##
The ratio of the density times the specific heat of 5 percent dextrose to the density times the specific heat of the blood.
C.sub.T =Correction factor for the injectate temperature rise through the catheter. PA1 60=Seconds/minute. PA1 V.sub.I =Volume of injectate in liters. PA1 T.sub.B =Initial blood temperature in .degree.C. PA1 T.sub.I =Initial injectate temperature in .degree.C. PA1 .intg.T.sub.B (t)dt=Area under the time-temperature thermodilution curve in .degree.C.-sec.
Careful analysis of the equation will show that to maximize accuracy of the analysis: (1) T.sub.I should be as low as possible to maximize the term (T.sub.B -T.sub.I); (2) T.sub.I must be known as accurately as possible. Initial methods to achieve these two objectives involved either: (1) prefilling syringes and soaking them in an ice bath (for up to one hour) and monitoring the temperature of the ice bath as an indication of the syringe temperature; or (2) drawing syringes from a sterile beaker full of fluid that has been allowed to sit in an ice bath until thoroughly chilled. Again the ice bath temperature was used as an indication of the injectate temperature.
As is apparent and readily appreciated, there exists a potential for significant inaccuracies in the value for T.sub.I. Differences between bath temperature (measured) and the syringe temperature (actual), combined with the fact that the syringe rapidly changes temperature after it leaves the ice bath, indicate the need to measure the injectate temperature precisely at the time of injection.
Other factors also dictate the need for an improved technique and assembly. Because the procedure may be performed in excess of 30 times on a single patient and each performance requires 3 to 5 separate injections, there is a significant need to reduce the potential for contaminating the patient with bacteria introduced while manipulating the solutions. Two critical times for such potential contamination are the steps involved in prefilling the syringes and the connection and disconnection of the syringes to the catheter at the time of use. Because the syringes and ice bath must be prepared 45 minutes to one hour prior to performing the procedure to adequately chill the injectate, either a continuous supply of such prefilled syringes must be maintained for emergencies or the test must be delayed.
It would be desirable to provide an assembly which would overcome the various shortcomings indicated for the current systems. It would be desirable to provide an assembly which would be ready to use immediately upon charging the cooling bath with ice; wherein the rate at which chilled solution can be withdrawn is significantly greater than current coil designs; which could be pre-chilled, removing the warm solution from the lines, without opening the system to contamination; wherein temperature of injection T.sub.I is maintained nearly at 0.degree. C. up to the catheter injection port; and which includes a rugged, totally disposable, in-line temperature probe.